About 9,000 non-military outdoor shooting ranges exist in the U.S. alone, with millions of pounds of lead from bullets shot every year. Some of the world’s most heavily contaminated soils can be found at firing ranges, Science reports, and researchers have now found a way to sop up those metals using cheap waste byproducts.

Lead and copper from spent bullets can leach into the ground, threatening water supplies, killing microbes, and poisoning plants, Science explains. Not to mention, workers and users of outdoor firing ranges may be exposed to hazardous levels of lead, according to the U.S. Centers for Disease Control and Prevention. These include about a million federal, state, and local law enforcement officers who are required to train with firearms, and another 50,000 shooting range workers. Cleaning the soil, however, is usually too expensive.

So, a team of researchers led by Deok Hyun Moon from Chosun University in Korea devised a cost-effective treatment scheme to stabilize lead and copper in contaminated soil using natural, renewable waste resources. Specifically, they turned to waste oyster shells and fly ash.

Landfills in Korea accumulate more than 250,000 tons of oyster shells each year, Science reports. Meanwhile, coal-fired power plants churn out just as much fly ash. Also called flue ash, these are the tiny particles of ash, dust, and soot that usually get carried away by the wind after they’re spewed out of smokestacks. Science explains:

Combining the two waste products creates a concoction rich in minerals that shackle metal ions within tight molecular bonds.

The researchers mixed their oyster shell and fly ash blend with firing-range soil that contained dangerous levels of lead. About a month later, the treatment led to a significant reduction in the “leachability” of the metals: 98 percent for lead and 96 percent for copper.